This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
In order for terminals to properly communicate via a wireless communications network including radio access nodes, it is necessary to establish and maintain synchronization among the radio access nodes.
Some of the wireless communications networks currently deployed, such as Time Division Long Term Evolution (TD-LTE), Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) and Code Division Multiple Access 2000 (CDMA2000) networks, rely on the Global Positioning System (GPS) to achieve synchronization. To be specific, each radio access node in the network calibrates its local timing by referring to a GPS reference signal. As a result, all radio access nodes in the network are synchronized to each other.
Additionally or alternatively, various other approaches than GPS may be employed to achieve synchronization among access nodes in wireless communications networks. For example, in Universal Mobile Telecommunications System Terrestrial Radio Access Networks (UTRANs), a Radio Network Controller (RNC) maintains a time reference and Base Stations (BSs) controlled by the RNC synchronize to each other by referring to the time reference. In Wireless Fidelity (WiFi) networks, each Access Point (AP) adjusts its timing by referring to the fastest one of the clocks of other APs it perceives.
To achieve the ultimate goal of mobile broadband which should be the ubiquitous and sustainable provision of non-limiting data rates to everyone and everything at every time, Ultra Dense Network (UDN) has been proposed which is characterized by sufficient provision of APs and operation at very wide bandwidths in the millimeter-wave bands.
Instead of wired connections arranged between radio access nodes in traditional cellular networks for those nodes to communicate with each other, wireless backhauls are provided in UDNs to support flexibility in the deployment of APs while minimize the cost for the deployment. Accordingly, wireless backhaul based synchronization schemes have been particularly proposed to achieve time synchronization as well as frequency synchronization among APs in the UDNs.
According to an example of the existing wireless backhaul based synchronization scheme as illustrated in FIG. 1, one of multiple APs in a UDN (denoted as AP0) which can receive an external synchronization signal from a satellite selects the satellite as an external synchronization source and synchronizes to the synchronization source by referring to the external synchronization signal. Meanwhile, AP0's neighboring AP (denoted as AP1) selects AP0 as its local synchronization source and synchronizes to AP0 by referring to a synchronization signal transmitted by AP0. Likewise, another one of AP1's neighboring APs (denoted as AP2) selects AP1 as its local synchronization source and synchronizes to AP1 by referring to a synchronization signal transmitted from AP1, and so on. In this manner, a synchronization chain is formed from AP0 to APn (n≥2) along which inter-AP synchronization signals are propagated and AP1 to APn are synchronized to AP0 directly or indirectly by referring to the respective inter-AP synchronization signals.
Although many solutions to the problem of how an AP may determine its local synchronization source have been proposed in the prior art (see CN102440031A and CN102625439), there is lack of a mechanism that ensures proper propagation of inter-AP synchronization signals among APs for maintaining synchronization among the APs.